Paul, do you mortar your rocks together?
Roots can break up rocks I have a photo of the roots of a encina Spanish evergreen oak quercus ilex subspecies ballota or rotundifolia, I took in a quarry and the roots made a vertical crack in the rock face. The rocks seemed to run in horizontal layers where there were no trees, I trespassed in the weekend to get the photos. The cracks the roots lived in where all earthy looking and wetter looking slightly darker than the next door rock. It seems the roots had crumbled the rock, a book i have on encinas says water collects in the cracks their roots bore in the ground.